The present invention is directed to a method for imaging a printing form using at least one controllable light source, a number of image spots being generated at a number of positions on the printing form in accordance with image data in a bit field, by the controlled action of light on the printing form. The present invention is also directed to a device for imaging a printing form, having at least one controllable light source and a control unit including a processor and a memory unit.
When imaging printing forms, printing-form precursors and printing plates in a print unit of a printing press, errors can arise due to unintentional relative motion, in particular vibrations, between the imaging device and the printing form, the printing form being accommodated, in particular, on a cylinder or constituting the surface area of a cylinder. This unintentional relative motion is caused, for example, by vibrations, by kineostatic deformations of the side frames, or by bearing inaccuracies. An error resulting therefrom is particularly evident in solid areas, in the form of small, narrow, unimaged hairlines (setting lines) between two adjacent imaged lines of image spots that can be produced, in particular, by light from two different light sources. In other words, the actual spacing of adjacent lines of image spots can deviate from the nominal spacing.
Practical approaches for taking constructional or mechanical steps to reduce the unintentional relative motion are often relatively complex and are associated with substantial costs: One step can entail, for example, uncoupling individual machine elements, such as the inking system, or in the case of sheet-processing printing presses, the feeder or the delivery unit, or even the complete press, from a cylinder provided with an individual drive and bearing the printing form. Another step can involve optimizing the mechanics necessary for the imaging device with respect to vibrational response, for example by providing a high degree of stiffness, substantial damping action, or a low weight. However, because of the often broadband vibrational spectrum and the small tolerable amplitudes, all unintentional relative motion is not able to be reliably avoided or eliminated in this way by shifting natural frequencies to higher frequencies. Also, such measures cannot compensate for kineostatic deformations or bearing inaccuracies.
Another approach for avoiding hairlines can provide for imaging using such large image spots that adjacent lines of image spots partially overlap. In the case of an (inadvertent) variation in the spacing of adjacent lines of image spots, which is small in comparison to the diameter of the image spots, the fact that they either still overlap one another or directly border on one another has the effect that no hairline is visible, in spite of (inadvertent) relative motion. However, when imaging using, in principle, larger image spots, this leads to limitations in the sparse regions in the printing image, thus in regions having low area coverage. In particular, gradations can then no longer be uniformly resolved.
From European Patent Application No. 1 176 545 A2, hereby incorporated by reference herein, it is known that images can be recorded on a printing form using at least one light source, the size of the image spots being variable. A number of image spots is produced at a number of positions on the printing form in accordance with image data in a bit field. The active spot size of the light beam striking the printing form surface, thus the area where the intensity exceeds the threshold for recording images on the printing form, is modifiable by varying the power of the light source (input power or optical power) or by varying the exposure time. The size of the image spots is changed as a function of the distance of the light source from the surface of the printing form. An increase in the energy introduced into the printing form leads to an enlargement of the image spot.
European Patent Document No. 0 734 151 B1, hereby incorporated by reference herein, describes a size-modulated, stochastic, rastered screening process. Raster points (halftone dots) are made up of a number of micro-dots or recorder elements (image spots). Micro-dots are the smallest addressable unit. From the raster points, raster areas are formed. The size of a particular raster point is modulated by varying the number of micro-dots as a function of the number of raster points surrounding the particular raster point.